Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing better image of the subsurface is an ongoing process.
For a seismic gathering process, as shown in FIG. 1, a marine seismic data acquisition system 100 includes a survey vessel 102 towing a plurality of streamers 104 (one shown) that may extend over kilometers behind the vessel. One or more source arrays 106 may also be towed by the survey vessel 102 or another survey vessel (not shown) for generating seismic waves 108. Conventionally, the source arrays 106 are placed in front of the streamers 104, considering a traveling direction of the survey vessel 102. The seismic waves 108 generated by the source arrays 106 propagate downward and penetrate the seafloor 110, eventually being reflected by a reflecting structure 112, 114, 116, 118 at an interface between different layers of the subsurface, back to the surface 119. The reflected seismic waves 120 propagate upward and are detected by detectors 122 provided on the streamers 104. This process is generally referred to as “shooting” a particular seafloor 110 area.
One of the shortcomings of existing technology relates to the poor azimuth/offset distribution of the data collection points, i.e., detectors 122, positioned along streamers of equal length, and the number of streamers 104 attached to the survey vessel 102. Generally, a single survey vessel 102 tows approximately ten to sixteen streamers 104, of uniform length, with detectors 122 equally spaced along the length of each streamer. In this configuration, the azimuth of the collection points is narrow. The azimuth is defined as the angle made between a line that passes through the source and a recording receiver and the navigation path when viewed from above the source and the recording receiver. Narrow azimuth distribution (typical for a single vessel seismic survey) leads to problems associated with multiple (reflective) removals at locations on the streamers in close proximity to the source arrays 106. It should be noted that a survey vessel is limited in the number of streamers 104 it can tow.
Another shortcoming associated with existing acquisition methods relates to the collected data in relation to its intended use, i.e., different streamer collection configurations lend themselves to different uses of the data, such as multiple removal, imaging and model building. Narrow azimuth distribution streamer configurations are not focused on a specific use of the collected data, resulting in less than optimal seismic image results.
An acquisition method having better azimuth and offset distribution than the system shown in FIG. 1 is illustrated in FIG. 2 (which corresponds to FIG. 11 of U.S. patent application Ser. No. 13/748,062, the entire content of which is incorporated by reference herein), in which a seismic survey system 200 includes five vessels 202 to 210. Vessels 202 and 204 are configured to tow corresponding streamer spreads 212 and 214, respectively, and corresponding source arrays 202a and 204a, while each of vessels 206, 208 and 210 are configured to tow only a corresponding source array 206a, 208a and 210a, respectively. This system has the vessels 202 to 210 distributed along a straight line 220 so that each vessel has a different inline direction (traveling direction) at a given instant.
The azimuth distribution associated with seismic acquisition system 200, which is schematically shown in FIG. 3A, is illustrated in FIG. 3B, which exhibits holes 318. FIG. 3C shows the offsets associated with the system of FIG. 3A. FIG. 3B plots the azimuth (grey scale) versus the Y and X offsets between the sources and sensors while FIG. 3C shows the number of events (on the Y axis) for a given offset versus the offset (on the X axis).
Accordingly, it would be desirable to provide systems and methods that further reduce the azimuth gaps and provide a higher offset count of the collected seismic data.